1. Field of the Invention
The invention is directed to a highly sensitive spectrum analysis unit which uses a diffraction grating, wherein a parallel light bundle having a wavelength range impinges on a diffraction grating which splits the different wavelengths in spectra by diffraction in first directions. The wavelength partial ranges of the spectrally split light bundle are focused onto a detector row through optics, and evaluating electronics connected to the detector row obtain the generated spectrum as information and display it.
The spectral unit can be applied in all spectrometers. The unit is used in particular in a confocal laser scanning microscope (LSM) such as is described in DE 197 02 753 A1 or DE 101 51 217 A1 as a measuring device for spectrally split detection of fluorescence.
2. Description of Related Art
There are a great many arrangements for spectral analysis of radiation which make use of a variety of dispersive means. Gratings and prisms are used most commonly. Simple solutions can also be realized with filters. Most applications use gratings with a surface profile whose topography is reproduced by replication methods permitting an economical production of unit quantities. Since generally only one diffraction order of the grating is used, the user has the possibility of obtaining much diffracted light in the useful order from sawtooth profile shapes. For optimal profile depth (approximately half of the wavelength of the diffraction maximum—blaze wavelength), between 70% and 90% diffraction efficiency is obtained in the reflected useful order depending on the spatial frequency of the grating. However, this diffraction efficiency decreases steadily when the blaze wavelength is abandoned for higher or lower wavelengths. This “missing light” in the spectrum of the useful order reaches other diffraction orders and escapes detection.
A known method for circumventing this problem is to use echelle systems as is described in EP 442 596 B1. A plane grating in very high diffraction orders is used for this purpose, wherein short, overlapping spectral regions of the different diffraction orders occur initially, all of which work in the vicinity of the blaze angle. A prism working perpendicular to the dispersion direction of the grating is added to the system for lateral separation of the spectra. Many order lines which are located one above the other laterally are obtained in this way. However, application of this method is conditional upon the use of a surface receiver. The reason for the spectral fluctuation in diffraction efficiency is that the electromagnetic behavior of the grating diffracts a more or less large proportion of the light in diffraction orders other than the desired, first diffraction order, and this light is accordingly lost and even causes scatter light problems.
There are some measurement methods in which the existing light in the spectrum (of one order) must be evaluated as completely as possible. One such method is the spectral evaluation of fluorescent light in microscopy with biological samples.